System and method of battery capacity reporting

ABSTRACT

A method and system for accurately reporting battery capacity is disclosed herein. The disclosed method and system prevent the reporting of discontinuous capacity values resulting from starting or stopping recharge cycles. The disclosed method and system prevent over or under reporting of battery capacity due to the transition between charge and discharge curves in a battery model.

FIELD OF THE INVENTION

The present invention relates generally to batteries. More particularly,the present invention relates to reporting of the capacity of a battery.

BACKGROUND OF THE INVENTION

Many mobile computing and communicating devices rely upon standardbattery cells for providing power on which to operate. Though disposablebattery cells, such as alkaline cells, are a well-known and reliabletechnology, it is common in such mobile devices to employ rechargeablebattery cells. These rechargeable batteries depend on a number of knowncell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cellsare known to those of skill in the art, as are some of theirdeficiencies. One of the known deficiencies of the above mentionedrechargeable battery cells is related to the fact that each battery hasa finite life span that can be measured in terms of recharge cycles. Theprocess of charging and discharging the cell damages the cell's chargestorage capabilities, causing the stored potential, which is typicallymeasured in mA-hours, to decrease over the life of the battery. As theability to store charge decreases, so does the battery's utility. Thelife of the battery can be drastically curtailed by improperly charging,or over discharging the battery. Another known deficiency of the abovecell types is that the batteries are known to discharge while instorage, though some types of battery are more susceptible to theself-discharge phenomenon than others. As a result of thesedeficiencies, it is crucial that a user be able to determine thecapacity of a battery both prior to and during use.

A state of the art technique for battery capacity reporting relies onthe coulomb counter. The principle of operation involved in coulombcounting is computing a coulomb count equal to the coulombs injectedinto a battery minus the coulombs taken out of the battery. The capacityof the battery is then reported by comparing the coulomb count relativeto a reference coulomb count value that corresponds to maximum batterycapacity. For instance, if the coulomb count of a battery is half of thereference value, the battery capacity is reported to be 50 percent.Although the coulomb counter addresses battery capacity reporting, itmay have several problems. First, the reported capacity may not bemeaningful if an accurate reference coulomb count value corresponding tomaximum battery capacity is not known. Furthermore, with a coulombcounter it may be difficult to keep an accurate reference coulomb count,particularly when battery capacity decreases over the lifetime of thebattery. Further still, with a coulomb counter it may be necessary toknow the current battery capacity before beginning the coulomb count.

A limitation of the coulomb counting principle is that it may not beapplicable to reporting the capacity of a battery of initially unknownbattery capacity: if the capacity of a battery is to be reported usingthe coulomb count system and method, the battery may have to be takenfrom it's unknown capacity state to either a fully charged 100 percentbattery capacity state or to a fully discharged 0 percent capacity statebefore the coulomb count can be used. Because the state of the batteryis unknown at a certain point, the only way to charge the battery to100% capacity is to constantly provide charge over an extended length oftime. This can result in an overcharging of the cell, which is known todamage to the storage capability of the cell. Conversely, to guaranteethat the cell is at 0% capacity, the cell must be completely discharged.It is a known phenomenon that rechargeable batteries are damaged by afull discharge to a complete empty state. Thus forcing a battery toeither 100% or 0% capacity will likely damage the cell, which onlyhastens the time at which the coulomb counting becomes inaccurate.

Further practical limitations exist with coulomb counting techniques. Inpractice, coulomb counting works by applying an integration over time.The presence of an offset in a coulomb counter may result in theinaccuracy of the coulomb count. This applies even to batteries with anassumed initially known battery capacity, and is compounded with everyrecharge cycle. This may be especially true if the battery needs to beused for a long period of time between opportunities to reset thecoulomb counter. For instance, in a battery that needs to be used for 3weeks between charges, even small offsets with each charge cycle mayaccumulate to large inaccuracies in reported capacity.

Other known techniques of battery capacity reporting exist, and areprimarily based on measuring battery voltage. The interest in suchvoltage techniques is due to the technical ease involved in voltagemeasurement. However, voltage measurement techniques also present thegreatest challenges since the relationship between battery voltage andbattery capacity is plastic, i.e. for any given battery capacity, themeasured battery voltage can vary greatly. The presence of suchvariations prevent the systematic reporting of meaningful batterycapacity values. The variations are small if the current draw is fairlyconstant over the lifetime of the battery, so there are situations wherea direct voltage to capacity mapping will suffice.

Many battery capacity reporting solutions assume a fairly constantcurrent draw for the major mode of operation, and only report capacityin this mode. For example, most cell phones only report battery capacitywhen they are not charging. Once they start charging, their batterygauges stop indicating battery capacity. However, in applications wherea battery is recharged while the system is running, such a change instate from discharging to charging, or vice versa, may break anyassumptions about constant current draw.

Batteries have known characteristic charge and discharge curves. FIG. 1illustrates a charge curve 140 and a discharge 130 curve for a battery.These curves relate battery voltage 120 to percent capacity 110 for arechargeable battery. The curves provide a model 100 for a battery. Inthe model, percent battery capacity 110 is related to battery voltage120 in either a discharging state, shown by discharge curve 130, or thecharging state shown by charge curve 140. Illustrated is a multiplicityof points such as point 132 on the discharging curve 130 and of point142 on the charging curve. Interpolation can be used to provide capacityvalues 110 for voltages 120 that lie between points for which values areknown.

In reference to FIG. 1, the details of a charge state capacity model 100are described. The relationship between battery voltage 110, batterycharge state and capacity 120 is illustrated by two curves 130,140. Afirst curve 140 corresponds to a positive battery charge current orcharging battery charge state, and a second curve 130 corresponds to anegative battery charge current or discharging battery charge state.

Although not expressly shown in the drawings, the charge state capacitymodel 100 can use more than one pair of curves. Each curve is a functionof both the battery charge current and the battery charge state. Thecharge state is used to select at least one curve from a multiplicity ofcharge curves. Each curve is a function of the battery charging current,and relates battery voltage to capacity. For example, when the batteryis in a first charge state, such as the charging state, a first chargecurve corresponding to the charging state is utilised. When the batteryis in a second charge state, for instance the discharging state, asecond charge curve corresponding to the discharge state is utilised.The charge curves are such that given a battery voltage value and acharge curve, it is possible to obtain a corresponding capacity valuefrom the charge curve.

Though it is possible to determine the capacity of a battery bymeasuring the voltage of the battery and examining the model, it shouldbe noted that the existence of two distinct curves presents a problem.When a battery is charging and is at 50% capacity, it has a definedvoltage level. If the battery charging is terminated when the battery isat 50%, the voltage of the battery does not instantly decrease to thevoltage that corresponds to 50% capacity on the discharge curve. Insteadthe voltage decays to that level over time. The voltage of a 50% batteryin a charging state is equivalent to the voltage of a 60-70% battery inthe discharging state. As a result, most voltage based battery capacityreporting devices report a capacity jump when charging is ended.Similarly, there is a reported battery capacity drop when charging isstarted. These abrupt changes in capacity are inaccurate, and causeconfusion among users.

There remains a further need for a system and method of battery capacityreporting based on battery voltage that overcomes the limitationspresent in the plastic relationship between battery voltage and batterycapacity.

There remains a further need still for a system and method of batterycapacity reporting which systematically reports a meaningful batterycapacity value whether the battery is being discharged or charged, andwhich does so regardless of the presence of transitions between thecharging and discharging of the battery.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous battery capacity reporters. It is a furtherobject of the present invention to provide a system and method forbattery capacity reporting based on battery voltage that is robustagainst inaccuracies in initial battery capacity estimations and whichsystematically provides a meaningful reported battery capacity value.

In a first aspect, the present invention provides a method ofdetermining the available battery capacity of a battery. In the method,a battery voltage and a current charge state of the battery aredetermined. These determined values are then used to determine a targetbattery capacity. The determined battery capacity is compared to aprevious battery capacity, and the target battery capacity is adjustedif the comparison is not indicative of the determined charge state. Inan embodiment of the present invention, the method further includeseither or both of the steps of reporting the target battery capacity andstoring the reported capacity as the previous battery capacity.

In a further embodiment of the first aspect of the present invention thetwo defined charge states are a charging state, and a discharging state.In the charging state, a target battery capacity less than the previousbattery capacity is not indicative of the charge state, while a targetbattery capacity greater than the previous battery capacity is notindicative of the discharging state. In a further embodiment,determining the battery capacity is done by examining a predeterminedmodel of the correlation between voltage, charge state and capacity.

In other embodiments of the present invention adjusting the targetcapacity can involve changing the target capacity to the value of theprevious battery capacity value or changing the target capacity to acapacity determined from a predefined fast transition curve that modelsthe relationship between the determined battery voltage, the determinedcurrent charge state and battery capacity. In a further embodiment tothe first aspect of the present invention, there is provided , prior tothe step of reporting, an adjustment step for adjusting the targetcapacity to a capacity determined from a predefined slow transitioncurve. The slow transition curve models the relationship between thedetermined battery voltage, the determined current charge state andbattery capacity, when the target capacity is in a play region aroundthe capacity of the battery when the last change in charge stateoccurred.

Further aspects of the first aspect of the present invention provide afurther adjustment of the target battery capacity based on an effectiveserial resistance correction factor or to compensate for temperaturefluctuations.

A second aspect of the present invention provides a system fordetermining the capacity of a battery with a memory for storing aprevious battery capacity value. The system has voltage reading means,charge state determining means, target capacity determining means, acomparator and target capacity adjusting means. The voltage readingmeans are operatively connected to the battery to determine the voltageof the battery. The charge state determining means are operativelyconnected to the battery to determine the charge state of the battery.The target capacity determining means, are operatively connected to thevoltage reading means to receive the determined voltage and to thecharge state determining means to receive the determined charge state,so that they can compute a target battery capacity based on thedetermined voltage and the determined charge state. The comparator isoperatively connected to the memory to receive the previous batterycapacity value and to the target capacity determining means to receivethe target battery capacity, it generates a comparison signalrepresentative of the comparison of the previous battery capacity valueand the target battery capacity. The target capacity adjusting means areoperatively connected to the comparator to receive the comparisonsignal, to the target capacity determining means to receive thedetermined target battery capacity and to the charge state determiningmeans to receive the determined charge state. The target capacityadjusting means adjust the determined target battery capacity if thecomparison signal is not indicative of the determined charge state, andthey also store the adjusted target battery capacity in the memory.

In an embodiment of the second aspect of the present invention there isprovided reporting means, operatively connected-to the target capacityadjusting means for reporting the adjusted target battery capacity.

In various embodiments, the target capacity adjusting means furtherincludes means for a number of functions. One such function is to adjustthe determined target capacity to a capacity determined from apredefined fast transition curve that models the relationship betweenthe determined battery voltage, the determined current charge state andbattery capacity after a change in charge state. Another such functionis to adjust the target capacity to a capacity determined from apredefined slow transition curve that models the relationship betweenthe determined battery voltage, the determined current charge state andbattery capacity when the target capacity is in a play region around thecapacity of the battery when the last change in charge state occurred.

In another embodiment the target capacity adjusting means is alsoconnected to an effective serial resistance tester which is operativelyconnected to the battery to determine an effective serial resistancecorrection factor, the target capacity adjusting means further includesmeans for adjusting the target capacity based on the effective serialresistance correction factor.

In a presently preferred aspect the above described system is integratedinto a handheld computing or communicating device.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying Figs.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figs., wherein:

FIG. 1 illustrates two curves, a charge and a discharge curve, relatingbattery voltage to percent capacity for a rechargeable battery, inaccordance with the present invention;

FIG. 2 is a block diagram of a mobile communication device in which theinstant invention may be implemented;

FIG. 3 is a flowchart illustrating a preferred embodiment of the methodof battery capacity reporting, in accordance with the present invention;

FIG. 4 is an enlarged version of a portion of FIG. 1, the portion boundby a dotted rectangle in FIG. 1;

FIG. 5 illustrates a transition from the use of the charge curve to theuse of the discharge curve of FIG. 4 in a first embodiment of a methodto carry out step 260 of FIG. 3, in accordance to the present invention;

FIG. 6 illustrates a transition from the use of the discharge curve tothe use of the charge curve of FIG. 4 in a first embodiment of a methodto carry out step 260 of FIG. 3, in accordance to the present invention;

FIG. 7 is a flowchart illustrating a first embodiment of a method tocarry out step 260 of FIG. 3, in accordance with FIGS. 5 and 6;

FIG. 8 illustrates a transition from the last reported capacity towardsthe discharge curve of FIG. 4 in a preferred embodiment of a method tocarry out step 260 of FIG. 3, in accordance to the present invention;

FIG. 9 illustrates a transition from the last reported capacity towardsthe charge curve of FIG. 4 in a preferred embodiment of a method tocarry out step 260 of FIG. 3, in accordance with the present invention;

FIG. 10 is a flowchart illustrating a preferred embodiment of a methodto carry out step 260 of FIG. 3, in accordance with FIGS. 8 and 9; and

FIG. 11 is a block diagram illustrating an exemplary embodiment of asystem of the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides a method and system formeasuring and reporting battery capacity.

FIG. 2 is a block diagram of a mobile communication device 10 in whichthe instant invention may be implemented. The mobile communicationdevice 10 is preferably a two-way communication device having at leastvoice or data communication capabilities. The device preferably has thecapability to communicate with other computer systems on the Internet.Depending on the functionality provided by the device, the device may bereferred to as a data messaging device, a two-way pager, a cellulartelephone with data messaging capabilities, a wireless Internetappliance or a data communication device (with or without telephonycapabilities). It will be apparent to one of skill in the art thatbatter capacity reporting and measurement has applications that are notlimited to the field of mobile communicating and computing devices.

Where the device 10 is enabled for two-way communications, the devicewill incorporate a communication subsystem 11, including a receiver 12,a transmitter 14, and associated components such as one or more,preferably embedded or internal, antenna elements 16 and 18, localoscillators (LOs) 13, and a processing module such as a digital signalprocessor (DSP) 20. As will be apparent to those skilled in the field ofcommunications, the particular design of the communication subsystem 11will be dependent upon the communication network in which the device isintended to operate. For example, a device 10 destined for a NorthAmerican market may include a communication subsystem 11 designed tooperate within the Mobitex# mobile communication system or DataTAC#mobile communication system, whereas a device 10 intended for use inEurope may incorporate a General Packet Radio Service (GPRS)communication subsystem 11.

Network access requirements will also vary depending upon the type ofnetwork 19. For example, in the Mobitex# and DataTAC# networks, mobiledevices such as 10 are registered on the network using a unique personalidentification number or PIN associated with each device. In GPRSnetworks however, network access is associated with a subscriber or userof a device 10. A GPRS device therefore requires a subscriber identitymodule (not shown), commonly referred to as a SIM card, in order tooperate on a GPRS network. Without a SIM, a GPRS device will not befully functional. Local or non-network communication functions (if any)may be operable, but the device 10 will be unable to carry out anyfunctions involving communications over network 19. When requirednetwork registration or activation procedures have been completed, adevice 10 may send and receive communication signals over the network19. Signals received by the antenna 16 through a communication network19 are input to the receiver 12, which may perform such common receiverfunctions as signal amplification, frequency down conversion, filtering,channel selection and analog-digital conversion. Analog to digitalconversion of a received signal allows complex communication functions,such as demodulation and decoding, to be performed in the DSP 20. In asimilar manner, signals to be transmitted are processed, includingmodulation and encoding for example, by the DSP 20 and input to thetransmitter 14 for digital to analog conversion, frequency upconversion, filtering, amplification and transmission over thecommunication network 19 via the antenna 18.

The DSP 20 not only processes communication signals, but also providesfor receiver and transmitter control. For example, the gains applied tocommunication signals in the receiver 12 and transmitter 14 may beadaptively controlled through automatic gain control algorithmsimplemented in the DSP 20.

The device 10 preferably includes a microprocessor 38 which controls theoverall operation of the device. Communication functions, including atleast one of data and voice communications, are performed through thecommunication subsystem 11. The microprocessor 38 also interacts withfurther device subsystems such as the display 22, flash memory 24,random access memory (RAM) 26, auxiliary input/output (I/O) subsystems28, serial port 30, keyboard 32, speaker 34, microphone 36, ashort-range communications subsystem 40 and any other device subsystemsgenerally designated as 42.

Some of the subsystems shown in FIG. 2 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 32 and display 22for example, may be used for both communication-related functions, suchas entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the microprocessor 38 is preferablystored in a persistent store such as flash memory 24, which may insteadbe a read only memory (ROM) or similar storage element (not shown).Those skilled in the art will appreciate that the operating system,specific device applications, or parts thereof, may be temporarilyloaded into a volatile store such as RAM 26. It is contemplated thatreceived communication signals may also be stored to RAM 26.

The microprocessor 38, in addition to its operating system functions,preferably enables execution of software applications on the device. Apredetermined set of applications which control basic device operations,including at least data and voice communication applications forexample, will normally be installed on the device 10 during manufacture.A preferred application that may be loaded onto the device may be apersonal information manager (PIM) application having the ability toorganise and manage data items relating to the device user such as, butnot limited to e-mail, calendar events, voice mails, appointments, andtask items. Naturally, one or more memory stores would be available onthe device to facilitate storage of PIM data items on the device. SuchPIM application would preferably have the ability to send and receivedata items, via the wireless network. In a preferred embodiment, the PIMdata items are seamlessly integrated, synchronised and updated, via thewireless network, with the device user's corresponding data items storedor associated with a host computer system thereby creating a mirroredhost computer on the mobile device with respect to the data items atleast. This would be especially advantageous in the case where the hostcomputer system is the mobile device user's office computer system.Further applications may also be loaded onto the device 10 through thenetwork 19, an auxiliary I/O subsystem 28, serial port 30, short-rangecommunications subsystem 40 or any other suitable subsystem 42, andinstalled by a user in the RAM 26 or preferably a non-volatile store(not shown) for execution by the microprocessor 38. Such flexibility inapplication installation increases the functionality of the device andmay provide enhanced on-device functions, communication-relatedfunctions, or both. For example, secure communication applications mayenable electronic commerce functions and other such financialtransactions to be performed using the device 10.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem 11and input to the microprocessor 38, which will preferably furtherprocess the received signal for output to the display 22, oralternatively to an auxiliary I/O device 28. A user of device 10 mayalso compose data items such as email messages for example, using thekeyboard 32, which is preferably a complete alphanumeric keyboard ortelephone-type keypad, in conjunction with the display 22 and possiblyan auxiliary I/O device 28. Such composed items may then be transmittedover a communication network through the communication subsystem 11.

For voice communications, overall operation of the device 10 issubstantially similar, except that received signals would preferably beoutput to a speaker 34 and signals for transmission would be generatedbased on an input received through a microphone 36. Alternative voice oraudio I/O subsystems such as a voice message recording subsystem mayalso be implemented on the device 10. Although voice or audio signaloutput is preferably accomplished primarily through the speaker 34, thedisplay 22 may also be used to provide an indication of the identity ofa calling party, the duration of a voice call, or other voice callrelated information for example.

The serial port 30 in FIG. 2 would normally be implemented in a personaldigital assistant (PDA)-type communication device for whichsynchronisation with a user's desktop computer (not shown) may bedesirable, but is an optional device component. Such a port 30 wouldenable a user to set preferences through an external device or softwareapplication and would extend the capabilities of the device by providingfor information or software downloads to the device 10 other thanthrough a wireless communication network. The alternate download pathmay for example be used to load an encryption key onto the devicethrough a direct and thus reliable and trusted connection to therebyenable secure device communication.

A short-range communications subsystem 40 is a further optionalcomponent which may provide for communication between the device 10 anddifferent systems or devices, which need not necessarily be similardevices. For example, the subsystem 40 may include an infrared deviceand associated circuits and components or a Bluetooth# communicationmodule to provide for communication with similarly-enabled systems anddevices.

A charging subsystem 44 is a component that provides power for thedevice 10 and different subsystems or devices. For example, the chargingsubsystem 44 may determine the presence of detachable power sourcedevice 46 and associated circuits, such as an AC adapter, USB bus, orcar adapter to provide power for the device and to charge battery 48.Additionally, charging subsystem 44 may determine the absence of powersource device 46, and consequently obtain power for the device 10 frombattery 48. When the battery 48 powers device 10, the battery 48 is saidto be in a discharging state. Conversely, when power source device 46powers device 10, and charging subsystem charges battery 48, the batteryis said to be in a charging state. The present invention is concernedwith reporting the capacity of a battery such as battery 48.

The battery capacity reported is a function of several factors,including battery voltage, and battery charging current. Therelationship between battery voltages, battery charging currents, andbattery capacity is modelled using charge curves such as thoseillustrated in FIG. 1. Therefore, before describing embodiments of themethod and system in detail, several concepts will be defined forgreater certainty.

As used in this description and in the appended claims, the batteryvoltage is defined as the voltage differential between positive andnegative terminals of the battery.

As used in this description and in the appended claims, the batterycharging current is defined as a current flow into the battery. Batterycharging current is capable of taking on a signed value, with a positivevalue meaning current being delivered into the battery and a negativevalue meaning current drawn out of the battery.

As used in this description and in the appended claims, charge state,also referred to as charging state, is defined as the sign of thecorresponding battery charging current. Therefore reference to apositive charge state is synonymous with charging. Similarly, a negativecharge state is synonymous with discharging. The use of either term isclear and unambiguous.

As used in this description and in the appended claims, a capacity modelis defined as the relationship between battery voltage, battery chargingcurrent, and capacity so that given battery voltage and battery chargingcurrent, capacity can be determined by applying the capacity model.

Generally, the method of the present invention adjusts the reportedbattery capacity to eliminate abrupt discontinuities in the reportedbattery capacity. The charging state of the battery is determined, andis used to select either the charge or the discharge curve. The voltageof the battery is then read, and using the selected curve a preliminary,or target, capacity is determined. The preliminary capacity is comparedto the last reported capacity. The comparison will show an increase inbattery capacity while the battery is in a discharge state if thecharging has been discontinued, or conversely will show a decrease incapacity while the battery is in the charge state if the charging hasbeen started. Because this is known to be inaccurate, an adjustment ismade in the preliminary battery capacity, and the adjusted capacity isreported. The reported capacity is then stored for use in the nextcycle. The method of adjustment of the battery capacity can be as simpleas reporting the previously reported value until the battery capacityfollows the known charge and discharge curves, or it can involve ananalysis of the reported voltage and a comparison of the reportedvoltage to a previously reported voltage to create a new curve throughwhich the battery capacity varies. The methods of the adjustment aredescribed in greater detail below.

Referring to FIGS. 1 and 2, in a preferred embodiment, the method uses asystem, such as device 10 of FIG. 2 including a charging subsystem 44,to assist in determining values for the battery voltage 120 and batterycapacity. The charging current can be used to determine the chargingstate and select either one of the curves 130,140. The chargingsubsystem 44 is typically capable of performing several operations suchas constant current charging operation, constant voltage chargingoperation, and no charging—or discharging—operation.

Referring now to FIG. 3, a flowchart illustrating the preferredembodiment of the method of battery capacity reporting, is described inreference to its steps.

At step 210, the battery voltage 120 is determined. At step 220, a model100 is provided, such as for example the model of FIG. 1. At step 230,the last reported capacity is provided. At step 240, a determination ismade as to the charging state of the battery. For instance if thebattery charging current is determined, the charging state can bederived from the sign of the charging current. Although not expresslyshown in the drawings, these first four steps can in any order, or canperformed simultaneously.

If at step 240, it is determined that the battery is charging, step 250Cis taken. Conversely, if at step 240, it is determined that the batteryis discharging, step 250D is taken. Step 250C selects the charge curve140 whereas step 250D selects the discharge curve 130. At step 260, thecharge curve model is applied to determine a capacity based on thedetermined battery voltage of step 210 and other factors.

Two embodiments of a method to carry out step 260 are currentlycontemplated. FIGS. 5-7 illustrate a first embodiment. FIGS. 8-10illustrate a second preferred embodiment which is easier to understandin view of the first. Both embodiments will be described in reference toFIG. 4.

FIG. 4 is an enlarged version of the dotted rectangular region 150 inFIG. 1. Shown is how the model 100 relates percent capacity 110 tobattery voltage 120 for two charge states, the discharge state curve 130with points 132 and the charge state curve 142.

In the charge state, the capacity model 100 uses an inherent property ofbattery charge current, the sign or charge state, to relate batteryvoltage to capacity as a function of charge state at step 260.

FIG. 5 illustrates a transition from the use of the charge curve 140 tothe use of the discharge curve 130 of FIG. 4 in a first embodiment of amethod to carry out step 260 of FIG. 3.

A battery 48 is assumed to be initially charging 140 and at voltage 120of 3.875 V, corresponding to point 142. Consequently, a 50% capacity 110is confidently determined. Next, the battery transitions to thedischarging state, for instance if power source 46 of FIG. 2 isdisconnected.

A battery that has been charging for a while and has a voltage readingof 3.875V, can be confidently gauged to be 50% full by directly mappingoff the initial charge curve, corresponding to a charging state. Ifcharging is turned off at this point, then the battery's voltage wouldhave to drop immediately to 3.825V in order for it to map to 50% on thenew charge curve, corresponding to a discharging state. However, what isobserved is that the battery voltage actually takes some time (forinstance tens of minutes, if not more than an hour) to settle to 3.825Vfrom 3.875V after charging has stopped. During that time, mapping thevoltage directly off the new charge curve 330D would yield a capacityvalue greater than 50%. If that value were reported directly, then theuser would see a reported battery capacity jump up to around 60% whenthe device 10 is disconnected from the charger 46.

Line D-D defines a discharge region 300D. Two possible transitionsbetween the charge and discharge curves are shown as transition 320D andtransition 330D relative to initial charge point 142. Transitions 330Dand 320D are illustrative only—several valid transitions such as 320Dand invalid transitions such as 330D can be defined. They all have incommon the fact that valid transitions 320D only allow the reportedcapacity to decrease when discharging, whereas invalid transitions 330Dcause the reported capacity to increase while discharging.

FIG. 6 illustrates a transition from the use of the discharge curve 130to the use of the charge curve 140 of FIG. 4 in a first embodiment of amethod to carry out step 260.

A battery 48 is assumed to be initially discharging 130 and at voltage120 of 3.825 V, corresponding to point 132. Consequently, a 50% capacity110 is confidently determined. Next, the battery transitions to thecharging state, for instance if power source 46 of FIG. 2 is connected.

A battery that has been discharging for a period of time and has avoltage reading of 3.825V can be confidently gauged to be 50% full bydirectly mapping off the initial charge curve, corresponding to adischarging state. If charging is turned on at this point, then thebattery's voltage would have to rise immediately to 3.875V in order forit to map to 50% on the new charge curve, corresponding to a chargingstate. However, what is observed is that the battery voltage willactually take some time (for instance tens of minutes, if not more thanan hour) to settle to 3.875V from 3.825V after charging has started.During that time, mapping the voltage directly off the new charge curve330C would yield a capacity value lower than 50%. If that value werereported directly, then the user would see a reported battery capacityjump down to around 30% when the device 10 is connected to the charger46.

Line C-C defines a charge region 300C. Two possible transitions betweenthe charge and discharge curves are shown as transition 320C andtransition 330C relative to initial discharge point 132. Transitions330C and 320C are illustrative only—several valid transitions 320C andinvalid transitions 330C can be defined. They all have in common thefact that valid transitions 320C only allow the reported capacity toincrease when charging, whereas invalid transitions 330D would cause thereported capacity to decrease while charging.

FIG. 7 is a flowchart illustrating a first embodiment of a method tocarry out step 260 of FIG. 3, in accordance to FIGS. 5 and 6.

System 10 provides the last reported capacity at step 410 and acandidate capacity at step 420. At step 430, a determination is made asto the charging state of battery 48, similar to step 240 alreadydescribed in reference to FIG. 3. If the battery 48 is in the chargingstate, then steps 440C, 450C or 460 are taken. Conversely, if thebattery is in the discharging state, then steps 440D, 450D or 460 aretaken.

If the battery 48 is in the charging state, at step 440C, the candidatecapacity provided in step 420 is compared to the last reported capacityprovided in step 410. If the candidate capacity is greater than the lastreported capacity, then at step 450C the candidate charge capacityprovided at step 420 is used. Conversely, if the candidate capacity isless than or equal to the last reported capacity, the last reportedcapacity is used at step 460. This ensures that only charge transitions320C of FIG. 6 occur, avoiding transitions of the type of 330C outsidethe charge region 300C.

If the battery 48 is in the discharging state, at step 440D, thecandidate capacity provided in step 420 is compared to the last reportedcapacity provided in step 410. If the candidate capacity is less thanthe last reported capacity, then at step 450D the candidate dischargecapacity provided at step 420 is used. Conversely, if the candidatecapacity is greater than or equal to the last reported capacity, thelast reported capacity is used at step 460. This ensures that onlydischarge transitions 320D of FIG. 5 occur, avoiding transitions of thetype of 330D outside the discharge region 300D.

According to the method of FIG. 7, the reported capacity is only allowedto increase when the battery is in a charging state. Similarly, thereported capacity is only allowed to decrease when the battery is in adischarging state.

When a change in charge state occurs, from the first initial chargestate to the second new charge state, it may take some time for thebattery to reach a new dynamic equilibrium at the second charge state.During this transition period, it is possible that neither the chargecurve corresponding to the initial charge state nor the charge curvecorresponding to the new charge state provides a sufficiently accuratevoltage-to-capacity mapping. For instance, in reference to FIGS. 5-6, atransition midway along line DD or CC would have a constant 50% lastreported capacity but could have a voltage of 3.850 V, a point that isneither on the charge curve nor on the discharge curve. This conceptleads to the preferred embodiment of a method to carry out step 260 ofFIG. 3, which will be discussed presently in reference to FIGS. 8-10.

FIG. 8 illustrates a transition from the last reported capacity 500towards the discharge curve 130 of FIG. 4 in a preferred embodiment of amethod to carry out step 260 of FIG. 3. As compared to FIG. 5, dischargearea 300D is still defined by line DD. A “fast” transition 520D replacestransition 320D. However, instead of avoiding the reporting of alltransitions 330D that might increase reported capacity, a smaller charge“play” area 510D is defined by line CC and “slow” transitions 530Dthrough the charge play area 510D are allowed. “Fast” and “slow” arerelative to one another so that their cumulative long-term effect is tofavour the reporting of capacity decreases when in the discharge state.For example, a “fast” transition might take 8.5 minutes to travel 80percent of the distance to the discharge curve 130 whereas a “slow”transition might take 34.3 minutes. Note that transitions 330D outsidethe play area 510D still do not cause a change in the reported capacity.

FIG. 9 illustrates a transition from the last reported capacity towardsthe charge curve of FIG. 4 in a preferred embodiment of a method tocarry out step 260 of FIG. 3. As compared to FIG. 6, charge area 300C isstill defined by line CC. A “fast” transition 520C replaces transition320C. However, instead of “banning” all transitions 330C that mightdecrease reported capacity, a smaller discharge “play” area 510C isdefined by line DD and “slow” transitions 530C through the discharge“play” area 510C are allowed. “Fast” and “slow” are relative to oneanother so that their cumulative long-term effect is to favour thereporting of capacity increases when the battery is in the charge state.For example, a “fast” transition might take 1 minute to travel 80percent of the distance to the discharge curve 130 whereas a “slow”transition might take 17.2 minutes. Note that transitions 330C outsidethe play area 510C still do not cause a change in the reported capacity.

FIG. 10 is a flowchart illustrating a preferred embodiment of a methodto carry out step 260 of FIG. 3, in accordance to FIGS. 8 and 9.

At step 610, “fast” and “slow” transition rates are provided by system10. These rates can differ depending on whether the battery is in acharge state or in a discharge state, as was described in reference toFIGS. 8 and 9.

At step 620, a target capacity is provided by the system 10. Preferably,the target capacity lies either on the charge curve 140 or the dischargecurve 130 depending on whether the battery is in a charge state or adischarge state, respectively.

At step 640, a “play” region is provided by the system 10. Preferably,the “play” region varies with the slope of the charge 140 or discharge130 curves, and is a function of the charge state. For instance, if thelast reported capacity while charging is less than 7%, a 1% wide playregion can be used, whereas if the last reported capacity is greater orequal to 7%, a 6% wide play region can be used. Similarly, if the lastreported capacity while discharging is greater than 10%, a 6% wide playregion can be used, whereas if the last reported capacity is smallerthan or equal to 10%, a 1% wide play region can be used.

At step 640, a determination is made as to the charging state of battery48, similar to step 240 already described in reference to FIG. 3. If thebattery 48 is in the charging state, then step 650C is taken, as well as660C or 670,680 or 690. Conversely, if the battery is in the dischargingstate, then step 650D is taken, as well as 660D or 670,680 or 690.

If the battery is in a charging state, at step 650C, the target capacityprovided in step 620 is compared to the last reported capacity. If thetarget capacity is greater than the last reported capacity, then at step660C a “fast” transition towards the charge target capacity ensues.However, if the target capacity is less than or equal to the lastreported capacity, then at step 670, the target capacity is checked withrespect to the “play” region. If the target capacity is within the playregion, then at step 680 a “slow” transition towards the charge targetcapacity ensues. However, if the target capacity is outside the “play”region, then at step 690 the last reported capacity is used.

If the battery is in a discharging state, at step 650D, the targetcapacity provided in step 620 is compared to the last reported capacity.If the target capacity is less than the last reported capacity, then atstep 660D a “fast” transition towards the discharge target capacityensues. However, if the target capacity is greater or equal to the lastreported capacity, then at step 670, the target capacity is checked withrespect to the “play” region. If the target capacity is within the playregion, then at step 680 a “slow” transition towards the dischargetarget capacity ensues. However, if the target capacity is outside the“play” region, then at step 690 the last reported capacity is used.

Although not expressly shown in the drawings, in another embodiment, acorrected battery voltage is computed before utilising the new chargecurve. In order to compute the voltage correction, a measured batterycurrent is taken from the battery. The value of the measured batterycurrent can be positive or negative, depending on the direction ofcurrent flow into or out of the battery.

Using an effective serial resistance (ESR) for the battery, a batteryvoltage correction term is obtained by multiplying the value of the ESRfor the battery and an estimated battery current. The corrected batteryvoltage is obtained by adding the battery voltage correction term to theestimated battery voltage while taking into account the direction ofcurrent flow in the addition. The estimated battery current can bedetermined by several ways, such as by measurement. The correctedbattery voltage is utilised with the new charge curve in order to find acorresponding capacity.

As used in this description and in the appended claims, ESR correctedcapacity reporting is defined as reporting a new capacity by correctingthe battery voltage based on ESR and an estimated battery current priorto determining the capacity based on the corrected battery voltage.

Furthermore, in yet another embodiment, in order to keep the reportedcapacity from transitioning too abruptly, the reported capacity isaffected with the value of the corresponding capacity progressively suchthat the reported capacity reaches the value of the correspondingcapacity at a convergence rate which is selected from a multiplicity ofconvergence rates comprising a “fast” convergence rate and a “slow”convergence rate. The determination of which convergence rate to use ismade as a function of the difference between the last reported capacityand the charge curve capacity, as well as the charge state of thebattery. As used in this description and in the appended claims,progressive capacity reporting is defined as reporting a new capacity bya progression from an initial capacity to the new capacity over time.

Although not explicitly shown in the drawings, temperature correctionscan be utilised throughout to ensure that the temperature of the batteryis also taken into account.

The above method is typically implemented as an embodiment of chargingsubsystem 44. The system includes means for determining the voltage ofthe battery and its present charge state. These means provide thedetermined values to means for determining the target capacity. Thetarget capacity is determined according to the methods described aboveand is then provided to a comparator, which compares the target capacitywith previous capacity. The result of the comparison is used by targetcapacity adjusting means to adjust the target capacity value. Theadjustment can use any combination of the methods described above toadjust the value of the battery capacity.

As illustrated in FIG. 1 the charging subsystem 44 has voltage readingmeans 700, charge state determining means 702, target capacitydetermining means 704, a comparator 706, whose functionality may beprovided by microprocessor 38, and target capacity adjusting means 708.The voltage reading means 700 are operatively connected to the battery48 to determine the voltage of the battery 48. The charge statedetermining means 702 are operatively connected to the battery 48 todetermine the charging state of the battery 48. The target capacitydetermining means 704, are operatively connected to the voltage readingmeans 700 to receive the determined voltage and to the charge statedetermining means 702 to receive the determined charging state, so thatthey can compute a target battery capacity based on the determinedvoltage and the determined charging state. The comparator 706 isoperatively connected to the memory 710, which may be flash memory 24,RAM 26 or another memory system, to receive the previous batterycapacity value and to the target capacity determining means 704 toreceive the target battery capacity. Comparator 706 generates acomparison signal representative of the comparison of the previousbattery capacity value and the target battery capacity. The targetcapacity adjusting means 708 are operatively connected to the comparator706 to receive the comparison signal, to the target capacity determiningmeans 704 to receive the determined target battery capacity and to thecharge state determining means 702 to receive the determined chargingstate. The target capacity adjusting means 708 adjust the determinedtarget battery capacity if the comparison signal is not indicative ofthe determined charging state, and they also store the adjusted targetbattery capacity in the memory 710. Optionally, there may also bereporting means 712, operatively connected to the target capacityadjusting means 708 for reporting the adjusted target battery capacity.

In various embodiments, the target capacity adjusting means 708 furtherincludes means for a number of functions. One such function is to adjustthe determined target capacity to a capacity determined from apredefined fast transition curve that models the relationship betweenthe determined battery voltage, the determined present charging stateand battery capacity after a change in charging state. Another suchfunction is to adjust the target capacity to a capacity determined froma predefined slow transition curve that models the relationship betweenthe determined battery voltage, the determined present charging stateand battery capacity when the target capacity is in a play region aroundthe capacity of the battery when the last change in charging stateoccurred.

In another embodiment the target capacity adjusting means 708 is alsoconnected to an effective serial resistance tester 714 which isoperatively connected to the battery 48 to determine an effective serialresistance correction factor, the target capacity adjusting means 708further includes means for adjusting the target capacity based on theeffective serial resistance correction factor. In embodiment of thepresent invention, the above described system is integrated into ahandheld computing or communicating device.

The above-described aspects of the invention provide a system and methodthat mitigate the uncertainty in battery capacity reporting resultingfrom the transition between the charge and discharge curves of thebattery model that are present in the prior art. Additionally thepresent invention accounts for the plastic relationship between batteryvoltage and battery capacity.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

1. A method of determining the available battery capacity of a batterycomprising: determining a battery voltage and a current charge state ofthe battery; determining a target battery capacity based on thedetermined battery voltage and the determined current charge state;comparing the determined target battery capacity to a previous batterycapacity; and adjusting the target battery capacity if the comparison isnot indicative of the determined charge state.
 2. The method of claim 1further including the step of reporting the target battery capacity. 3.The method of claim 1 further including the step of storing the reportedcapacity as the previous battery capacity.
 4. The method of claim 1,wherein the two defined charge states are a charging state, and adischarging state.
 5. The method of claim 4, wherein a target batterycapacity less than the previous battery capacity is not indicative ofthe charging state.
 6. The method of claim 4, wherein a target batterycapacity greater than the previous battery capacity is not indicative ofthe discharging state.
 7. The method of claim 1, wherein determining atarget battery capacity based on the determined battery voltage and thedetermined current charge state includes determining the capacity byexamining a predetermined model of the correlation between voltage,charge state and capacity.
 8. The method of claim 1, wherein the step ofadjusting includes changing the target capacity to the value of theprevious battery capacity value.
 9. The method of claim 1, wherein thestep of adjusting includes changing the target capacity to a capacitydetermined from a predefined fast transition curve that models therelationship between the determined battery voltage, the determinedcurrent charge state and battery capacity.
 10. The method of claim 1,further including, prior to the step of reporting, the step of adjustingthe target capacity to a capacity determined from a predefined slowtransition curve that models the relationship between the determinedbattery voltage, the determined current charge state and batterycapacity, if the target capacity is in a play region around the capacityof the battery when the last change in charge state occurred.
 11. Themethod of claim 1, further including the step of adjusting the targetbattery capacity, prior to the step of reporting, using an effectiveserial resistance correction factor.
 12. The method of claim 1, furtherincluding the step of adjusting the target battery capacity, prior tothe step of reporting, to compensate for temperature fluctuations.
 13. Asystem for determining the capacity of a battery having a memory forstoring a previous battery capacity value, the system comprising:voltage reading means operatively connected to the battery fordetermining the voltage of the battery; charge state determining meansoperatively connected to the battery for determining the charge state ofthe battery; target capacity determining means, operatively connected tothe voltage reading means for receiving the determined voltage and tothe charge state determining means for receiving the determined chargestate, for computing a target battery capacity based on the determinedvoltage and the determined charge state; a comparator, operativelyconnected to the memory for receiving the previous battery capacityvalue and to the target capacity determining means for receiving thetarget battery capacity, for generating a comparison signalrepresentative of the comparison of the previous battery capacity valueand the target battery capacity; and target capacity adjusting meansoperatively connected to the comparator for receiving the comparisonsignal, to the target capacity determining means for receiving thedetermined target battery capacity and to the charge state determiningmeans for receiving the determined charge state, for adjusting thedetermined target battery capacity if the comparison signal is notindicative of the determined charge state, and for storing the adjustedtarget battery capacity in the memory.
 14. The system of claim 13,further including reporting means, operatively connected to the targetcapacity adjusting means for reporting the adjusted target batterycapacity.
 15. The system of claim 13, wherein the target capacityadjusting means further includes means for adjusting the determinedtarget capacity to a capacity determined from a predefined fasttransition curve that models the relationship between the determinedbattery voltage, the determined current charge state and batterycapacity after a change in charge state.
 16. The system of claim 13,wherein the target capacity adjusting means further includes means foradjusting the target capacity to a capacity determined from a predefinedslow transition curve that models the relationship between thedetermined battery voltage, the determined current charge state andbattery capacity when the target capacity is in a play region around thecapacity of the battery when the last change in charge state occurred.17. The system of claim 13, wherein the target capacity adjusting meansis further operatively connected to an effective serial resistancetester which is operatively connected to the battery for determining aneffective serial resistance correction factor, and wherein the targetcapacity adjusting means further includes means for adjusting the targetcapacity based on the effective serial resistance correction factor. 18.A handheld device having a system for determining the capacity of abattery having a memory for storing a previous battery capacity value,the system comprising: voltage reading means operatively connected tothe battery for determining the voltage of the battery; charge statedetermining means operatively connected to the battery for determiningthe charge state of the battery; target capacity determining means,operatively connected to the voltage reading means for receiving thedetermined voltage and to the charge state determining means forreceiving the determined charge state, for computing a target batterycapacity based on the determined voltage and the determined chargestate; a comparator, operatively connected to the memory for receivingthe previous battery capacity value and to the target capacitydetermining means for receiving the target battery capacity, forgenerating a comparison signal representative of the comparison of theprevious battery capacity value and the target battery capacity; andtarget capacity adjusting means operatively connected to the comparatorfor receiving the comparison signal, to the target capacity determiningmeans for receiving the determined target battery capacity and to thecharge state determining means for receiving the determined chargestate, for adjusting the determined target battery capacity if thecomparison signal is not indicative of the determined charge state, andfor storing the adjusted target battery capacity in the memory.